part 2] ITS STEUCTURE AND EOCK-SUCCESSION. 161 



brought over the higher Allt-Ddu Beds. The Llangower thrust^ 

 as seen in section near Llangower, has an inclination of about 40° 

 south-eastwards, and close to the point where it branches off from 

 this line the inclination of the Cefn-ddwy-graig thrust appears to 

 be similar, but farther north its inclination becomes definitely 

 lower and more affected by the topography. At Pont-y-Ceunant, 

 despite the shattering of the rocks associated with it, the main. 

 effect is the concealment of some of the Allt-Ddu Mudstones. 



The Moel-fryn Displacement is a very extensive fault, 

 bringing about displacement of the rocks along a line which ex- 

 tends completely across the area mapped ; it is responsible for much 

 discontinuity in the beds of the Grelli-grin Calcareous Ash Series, 

 these being often faulted out, together with parts of the Allt-Ddu 

 Mudstones and the Rhiwlas Beds. The packing along this line is, 

 however, decidedly less than along the Llangower displacement, 

 and the inclination of the fault-plane seems to be at a lower 

 angle ; tearing is not a characteristic feature of the rocks above it, 

 except at the northern end of the district, where it is compara- 

 tively slight as at Grelli-grin or on Bryn-cut. 



The Ffridd Defaid Line has been traced for only a short 

 distance : it appears to cause concealment of some of the Lower 

 Ashgillian Beds, and is very clearly indicated on the north side of 

 the Hirnant valley; but on the south the country is at first heavily 

 wooded and then merges into the open moor, where exposures are 

 so few and far between that it is no longer possible to trace the^ 

 displacement. 



There seems to be fairly definite evidence of the decrease in the- 

 importance of the displacements east of Bala Lake, and this is 

 continued still farther eastwards, where along the line of the Hir- 

 nant valley, at the source of that stream, the beds merely plunge 

 down steeply without any break, the softer slates being apj^arently 

 squeezed between the Denbigh Grits and Flags on the one hand 

 and the hard Moel-fryn Sandstones on the other. All the major 

 lines of displacement can be traced fairly easily across country, 

 since they are generally marked by a feature in the landscape, this 

 being, however, occasionally emphasized by the nature of the rocks 

 affected ; thus, the Llangower thrust is marked by a well-defined 

 fault-scarp, which is more pronounced when the harder Grlyn-Gower 

 Sandstones are brought against the Allt-Ddu Mudstones. 



A smaller but clearly defined scarp also marks the line of the 

 Cefn-ddwy-graig thrust ; on the other hand, the Moel-fryn line is 

 more usually indicated by a depression, as it is on the pass 

 separating the valle^^s of the Dwynant and the Afon Glyn. 



The tear-faults associated with the major lines of displacement 

 deserve mention. The most conspicuous are those associated with 

 the Llangower thrust : of these, the Ty'n-y-mur and Ty'n-y-twll 

 tears both effect a very noteworthy shift of the Frondderw Ash,. 



Q. J. G. S. No. SIO."^ M 



